This invention relates to an economical plastic can and a process for preparing such a can.
Plastic cans are desirable for containing food products because it is possible to produce and process them at costs below those for metal cans. Furthermore, plastic cans are not subject to denting during handling, a problem with metal cans which can make them undesirable to the consumer. In addition, plastic cans may be made of transparent materials which can be used to display the contents.
However, plastic cans in the existing art have exhibited several problems. One problem arises when hot contents are inserted into the can and the can is sealed before the contents cool or where hot contents are added and the can is sealed and retorted without careful control of the pressure imposed on the can. If negative pressure (vacuum) in the can is generated, due to contraction or condensation of the contents, so that the external pressure exceeds the internal pressure by more than about 28 kPa (4 psig), inward collapse of the can walls can occur. Metal cans are much less sensitive to this phenomenon. Strengthening the walls by adding more plastic, in an attempt to avoid this problem, can make the cans economically unattractive.
Most plastic cans in commerce have been designed for use under little pressure differential under ambient conditions. An example is oil cans, which are filled and sealed near room temperature. Although some plastic cans have been designed with a deformable molded end which will accommodate compressive forces, such cans are expensive to make and cannot compete commercially with conventional metal cans. There is thus a need for an inexpensive extruded can which can withstand significant compressive stress without deforming and which readily accepts standard metal ends to form a reliable, hermetic, retortable seal.
U.S. Pat. No. 4,655,682 discloses a hot fillable, collapse resistant polyester container. If desired the sidewall may be provided with internally directed vertically collapsible beads to accommodate a decrease in volume with the container as the product cools. The beads are formed by internal ribs of a blow mold.
U.S. Pat. No. 3,971,173 discloses a process for heat treating plastic bottles to reduce their creep characteristics.
U.S. Pat. No. 3,410,939 discloses a straight-walled, monolayer, polypropylene can with two metal ends double seamed to the flanged can end. The body is cut from an extruded tube and the flanges are cold-formed. The body wall is about 0.79-0.89 mm (31-35 mils) thick. The flange is "something less," tapered, and oriented about 45 degrees to the body. The can is designed to contain oil.
U.S. Pat. No. 3,491,936 discloses a straight-walled, monolayer, plastic can, injection molded with one molded end. The opposite end has a metal end double seamed to the flange. The body wall has a shoulder and a 50% reduced thickness wall near the flanged end. The body wall at the flanged end and the flange are the same thickness and the flange is straight and 90 degrees to the body. The can is designed to contain ham.
U.S. Pat. No. 3,507,086 discloses a straight walled, monolayer, extruded, thermoplastic can with a flange formed by blowing into a stationary or moving mold. The flange is the same thickness as the body and is preferred to be at 45 degrees to the body although semicircular or perpendicular flanges are possible. Metal ends are crimped or rolled onto the flanges. The can is designed to contain oil.
U.S. Pat. No. 3,538,595 discloses a straight-walled, monolayer, ethylene copolymer or polypropylene thermoplastic can extruded as a tube, shock cooled, and cut to stress-form a flange. The body and flange are about 1 mm (40 mils) thick and metal ends are crimped to the 2 mm (80 mil) wide outwardly flared flanges. The can is designed to contain oil.
U.S. Pat. No. 4,559,197 discloses a straight-walled, monolayer, biaxially drawn polyethylene terephthalate can which has been annealed. The 90 degree flanges are formed by dies with the can ends heated to a temperature above the glass transition temperature, but below the heat-set temperature. The die can be a metal end for the can which is subsequently double-seamed. The can is designed to contain a processable food or beverage.
U.S. Pat. No. 4,587,075 discloses a straight-walled, monolayer, biaxially drawn polyethylene terephthalate can which has been heat set. The body is heat set at a temperature higher than the use temperature, but less than the glass transition temperature, while being restrained from shrinking. Metal ends can be seamed to the can ends. The can is designed to contain a processable food or beverage.
U.S. Pat. No. 4,667,454 discloses a straight-walled, multilayer polypropylene can injection blow molded or thermoformed with one molded end. The opposite end has a metal end double seamed to a molded flange. The body wall has a ridge near one or both ends and is about 0.79 mm (31 mils) thick with a flange about 0.64 mm (25 mils) thick at 77.5 degrees to the body. The molded (bottom) end of the can is deformable under the different pressures of hot filling and retorting to prevent paneling. The body is heat treated to a temperature higher than the use (sterilization) temperature. The can is designed to contain low-acid foods, such as vegetables that must be retorted.
Japanese publication 57-36611 discloses a method of molding flanges on a straight-walled monolayer polyethylene terephthalate extruded tube, which is then cut into cans, and metal caps are mounted on the ends. The flanges are formed by reheating sections of the extruded and cooled tube above the glass transition temperature and using a vacuum to suck the reheated section into a moving mold to produce a flared (triangular) flange shape. A rectangular shape is also possible. The can is designed to contain various drinks.
U.S. Pat. No. 3,981,663 discloses an apparatus for making high speed corrugated plastic tubing, comprising a pair of complementary upper and lower mold assemblies each comprising an endless chain of articulately interconnected mold blocks and means for guiding the mold blocks along an endless track. The mold blocks define a tubular mold having a corrugated wall. A hot extruded tube of thermoplastic material is expanded into molding engagement with the wall of the mold by means of pressurized air.
U.S. Pat. No. 3,891,773 discloses a method of making corrugated plastic pipe, including continuously extruding a tube of plastic material into a molding zone especially constructed to mold the tube into a series of successive closely spaced body-forming lengths of annularly corrugated pipe. The molded tube is severed to form the pipe into lengths.